1. Field of the Invention
The present invention provides an improved catheter for imaging structures within the body of a patient. More particularly, the present invention provides an imaging catheter which is pre-filled with a fluid suitable for the transmission of imaging energy. The improved catheter may also include a reservoir to hold a supply of fluid to replace fluid lost by evaporation and vapor transmission through the catheter material.
2. Description of the Background Art
Methods and apparatus for imaging structures within the body of a patient are described in U.S. Pat. Nos. 4,794,931 and 5,000,185 to Yock, U.S. Pat. No. 5,117,831 to Jang et al., and elsewhere. Imaging catheters of the type described include a transducer, typically an ultrasonic transducer, at the distal end of a flexible tubular member which constitutes the catheter body.
Ultrasonic energy produced by the transducer is readily transmitted by water or saline solution, as well as blood and other body fluids. Ultrasonic energy is not readily transmitted by air however. Therefore, any air present in the catheter in the region of the transducer must ordinarily be flushed out of the catheter prior to its use. Saline solution or sterile water is typically used for this purpose. The fluid flush must be sufficiently vigorous and thorough to ensure the removal of all air from the catheter. Even a very small bubble in the region of the transducer may prevent satisfactory imaging.
An imaging catheter of the type described is commonly furnished with a flush port or other opening at its proximal end through which fluid may be injected to accomplish the desired flush. This configuration is less than ideal for flushing air from the distal end however. The narrow distal tip makes it difficult to adequately flush the interior of the catheter to eliminate air bubbles at the distal tip in the vicinity of the ultrasonic transducer.
An alternative approach to flushing the catheter is described in U.S. Pat. No. 5,049,130 to Powell, the disclosure of which is incorporated herein by reference. The Powell patent discloses an imaging catheter having a port near its proximal end for injecting flushing fluid into the catheter. The distal end is equipped with a special sheath.
Normally, the sheath disclosed in the '130 patent closes off and seals the distal end of the catheter. The sheath is composed of an elastic material, however, which allows it to deform in response to internal pressure within the catheter. When fluid is injected into the proximal end of the catheter, the sheath deforms in response and a flow path opens through the sheath at the distal end of the catheter. Air within the catheter may thereby be flushed out of the distal end of the catheter through the sheath.
An alternate approach to achieving adequate flushing of the region around the transducer is disclosed in U.S. Pat. No. 5,002,059 to Crowley et al. This patent describes an imaging catheter in which the transducer is housed within a chamber at the distal end of the catheter. The chamber has a cylindrical plug formed of a silicone elastomer or equivalent polymer. A syringe needle may be inserted into the chamber through the plug so that fluid may be injected directly into the region housing the transducer.
Each of the catheters described above is less than ideal in that the physician is required to flush the catheter prior to its use. This procedure can be time consuming and it is often difficult to ensure that the catheter has been flushed sufficiently to remove all air from the vicinity of the transducer.
It would be desirable therefore to provide a catheter pre-filled with saline solution or another fluid suitable for the transmission of ultrasound or other imaging signals. It would further be desirable if such a catheter were provided with a reservoir to hold a supply of the fluid to replace fluid lost from the catheter, e.g., through evaporation. This would allow the storage of the catheter for extended periods prior to its use. It would further be desirable if the catheter were provided with means for applying pressure to force fluid from the reservoir into the catheter in the event fluid is lost from the catheter.